436 Prof. S. Lovdn on the Structure of the Echinoidea. " 



It appears from this that the interradia in the SpatangidEe are 

 enlarged principally by the growth of the individual plates, 

 very little by the addition of new plates near the vertical ])lates. 

 Every plate has a nucleus, which may often be recognized as 

 its umbo, and is surrounded internally by the curves of growth. 



If we examine a Mellitahexaj)ora Q'6 millims. in length side 

 by side with an adult of 85 millims., we find (besides the changes 

 in the interradia of the ventral side, which are a consequence of 

 the outgrowth of the ambulacra at their expense) that all the 

 interradia in the small individual have from 9 to 10 plates, and 

 in the large one 13 or perhaps 12 ; for it is scarcely possible to 

 ascertain how far the plate through which the periphery passes 

 is or is not divided into two by a suture. Here, also, the un- 

 paired interradiura is differentiated from the rest, although not 

 so much as in Brissopsis. The hiatus (" lunula "), which occurs 

 early in this interradium (all the otiiers are situated in the 

 ambulacra and have not yet made their appearance), is in the 

 young nearly circular, in the adult long and narrow ; and it 

 shifts its position during growth, so that in the former it is 

 bounded by the ventral plates 2 and 3, and by the dorsals 5, 

 6, and 7, but in the adult by the same ventrals (2 and 3) but 

 by the dorsals 6, 7, and 8 ; the dorsals approach the margin, 

 and there even, in some degree, become ventrals. The peri- 

 proctium, which, like the stoma, is much larger in proportion 

 in the young than in the adult, is round in the former and sur- 

 rounded in front by a narrow margin of plate 2, which gradually 

 disappears, so that finally plate 1 constitutes the anterior boun- 

 dary of the aperture, which is oval in the adult. The circumstance 

 that the number of pairs of plates in the interradia in the adult 

 only in a slight degree exceeds that in the youngest, is not difficult 

 to observe also in the Cassidulida? and regular Echinidffi ; whilst 

 in all the augmentation within the ambulacra is far more consi- 

 derable, and extraordinarily great in the petala of thelrregulares, 

 in which it is greatly multiplied. We soon ascertain that in all 

 Echinoidea the interradia and ambulacra grow and move in- 

 dependently, the former as the plated perisome, the latter as 

 fixed arms. 



In Brissoj)sis it is easy to see that the peripetalous fasciola 

 strikes over the same interradial plate in the adult as in the 

 young, over plates 4 and 5 in the frontals, 6 and 7 in the 

 laterals, and over the tenth plate of the unpaired interradium ; 

 and it keeps in both to the same ambulacral plate in the bivium, 

 namely the 14th or 15th, in the paired radii of the trivium to the 

 ninth or tenth, but shifts, apparently, in the unpaired ambula- 

 crum from plates 4 and 5 to plates 5 and 6. So also the 

 infraanal fasciola passes in young and old over plate 3 of the 



